Rhododendrons and azaleas have no tap roots. They survive
on very fine roots. Think of spider webs. If you were to go to a nursery
and tip one out of the pot, you would typically see a dense mass of hair-like roots.
These fine roots anchor the plant and provide its feeding system so there have to be
a lot of them.

Rhododendrons and azaleas are very shallow rooted (1-2 feet deep)
and the root zone may dry out during the hot
season even though deeper rooted plants show no signs of drought stress.
Therefore, rhododendrons should be well watered during the hot months, especially
the first year after planting when the roots have not yet gotten out of the original
root ball and into the surrounding soil.

After transplanting the roots take longer to grow out into the
surrounding soil than most other kinds of plants. They act like
temperamental children that don't want to leave the house. Because of this, the
newly planted rhododendron will get its water out of the original root ball and
if this ball is allowed to dry out it can be difficult to get it wet again.
One way to rewet a dry root ball is to place a dripping hose at the base of the
trunk of the plant and let it run for several hours. Unless you have a very
wet climate, doing this in addition to the regular watering the plant gets
during the first growing season will help get the plant off to a good start.

Spider webs won't move into crusty, hard soil. The texture
of the soil surrounding the rootball should allow easy penetration of the fine
roots. They can't compete with the big kids in the neighborhood, so don't
plant where there are other competing surface roots.

Rhododendrons and azaleas ideally grow in damp, never soggy
soil. Think of taking a wet sponge and squeezing it. The remaining damp
sponge is the nearly perfect air and water environment for rhododendron roots.
Too much water promotes root rot that can cause plant death. Too little
water, and it feels like a dry sponge, and the plant will suffer and be badly
damaged. Low available water in the root zone can cause leaves to sun burn.
Sun burn appears as splotches of brown, tan or off-white areas on the top of the leaf.

Some foliage droop is normal in dry weather, especially on warm
afternoons, but when leaves still show signs of drooping in early morning, the
plant is showing a need for water and should be irrigated. When air
temperatures go above 95°F (or even lower for alpine types), rhododendrons and
azaleas appreciate a misting to prevent desiccation of their foliage. In
cold climates, watering or misting of foliage during warm days in the spring or
on windy days when the roots are still frozen will help to keep rhododendrons in
good condition.

A year-round mulch of some type of organic matter is desirable to conserve
moisture and eliminate the need for cultivation. Because of their shallow
roots, little or no cultivation should be done around rhododendrons. Weeds
should be carefully pulled, or in extreme cases shaved off with a sharp hoe.
A fairly deep mulch of leaves, pine needles, chips, bark or other organic
material will practically eliminate weed growth. (Peat moss should not be
used as a mulch because it sheds water when it dries out.) The coarser the
mulch the better, as water and air are admitted while the mulch still retards
evaporation by providing shade and reducing wind velocity over the roots.
A mulch also helps to maintain a moderate soil temperature in the root zone.

For further information on watering rhododendrons and azaleas
consult the following Journal ARS articles: